The free radical theory of aging suggests that the biological process of aging results in increased oxidative stress in elderly humans. The ability of a cell to resist the damaging potential of oxidative stress is determined by a vital balance between generation of oxidant free radicals and the defensive array of antioxidants available to the cell. There are multiple antioxidant defense systems and of these, glutathione (GSH) is the most abundant intracellular component of overall antioxidant defenses. GSH, a tripeptide, is synthesized from precursor amino-acids glutamate, cysteine, and glycine in two steps catalyzed by glutamate cysteine ligase (GCL, also known as γ-glutamylcysteine synthetase, EC 6.3.2.2) and γ-L-glutamyl-L-cysteine:glycine ligase (also known as glutathione synthetase, EC 6.3.2.3), and GSH synthesis occurs de novo in cells.
Glutathione deficiency has been implicated in several diseases in humans including protein energy malnutrition in children, sickle-cell anemia, infection, neurological disorders such as Parkinson's disease, HIV infections, liver disease and cystic fibrosis. Evidence from several animal (Stohs et al., 1984; Farooqui et al, 1987; Liu et al., 2000) and human studies (Al-Turk et al., 1987; Matsubara et al., 1991; Lang et al., 1992; Samiec et al., 1998; Erden-Inal et al., 2002; Loguercio et al., 1996) suggest that concentrations of glutathione also decline with aging. GSH deficiency in aging is associated with an increased pro-oxidizing shift (Rebrin, 2008) leading to increased oxidative stress (Rikans and Hornbrook, 1997). These changes have been implicated in diseases of aging such as cataracts (Campisi et al., 1999; Castorina et al., 1992; Sweeney et al., 1998), age-related macular degeneration (Samiec, 1998), altered immune function (Fidelus and Tsan, 1987; Furukawa et al., 1987) and neurodegenerative disease (Liu et al., 2004), and in increased DNA damage (Hashimoto et al., 2008) at a molecular level. While the underlying mechanisms for aging-associated glutathione deficiency is not well understood, there are suggestions that perturbations in glutathione synthesis could be involved (Toroser and Sohal, 2007).
Two key mechanisms for the intracellular GSH deficiency are suppressed synthesis and/or increased consumption relative to synthetic capacity. To determine whether increased GSH consumption or suppressed synthesis was responsible for intracellular GSH deficiency in aging, the inventor used an established stable-isotope tracer method (Reid and Jahoor, 2000) to measure in vivo erythrocyte GSH synthesis in 8 young and 8 aged humans, before and after 14 days of supplementation of 2 key amino-acid precursors of glutathione synthesis, cysteine and glycine. Glutathione concentrations within erythrocytes, and oxidative stress and markers of oxidant damage within plasma, were also measured.
Other and further objects, features, and advantages will be apparent from the following description of the presently preferred embodiments of the invention, which are given for the purpose of disclosure.